Why is the pilot diameter of a counterbore so large?

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Brad Amos on 30/01/2017 20:51:37:

My ' version of reality', as regards recommended clearance sizes, comes from the widely-used 'Zeus' precision tables. I now understand that this clearance size for the screw is less than the hole size needed to accommodate the radius on the base of the screw head …

.

Brad,

Sorry if I was a little blunt in my response, but; as you now appreciate 5.1mm as 'clearance' is quite irrelevant to the size of pilot on a standard counterbore.

MichaelG.

.

P.S. forgot to mention … the reason I asked initially about Imperial or Metric is that the Imperial ones have much neater proportions [because the shape and tolerance of the cap-head screws is different] : Go Compare !!

Edited By Michael Gilligan on 30/01/2017 21:56:45

[Neil WyattModerator @neilwyatt]

I make my own out of silver steel so they can be any size I want

Neil

[Clive HartlandParticipant @clivehartland94829]

It would appear that the counterbore size is related to the size of the cap screw heads and clearance due to the hole the screw goes through. An example is the hole drilled say M4 clearance which will give 0.5 clearance then the counter bore is 4.8 dia giving 0.8 clearance for the head of the screw used. It is after all a system.

Clive

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Neil Wyatt on 30/01/2017 22:07:33:

I make my own out of silver steel so they can be any size I want

.

Well, obviously you do … you're the Editor of Model Engineers' Workshop.

[John Stevenson 1Participant @johnstevenson1]

AS Neil says, if it's so important then make your own.

A selection made from broken and chipped end mills and slot drill with just a simple rotary jig and a bench grinder.

Choose what sizes you want.

L to R countersink tool. Counterbore. Tool for trepasnning the broken bit of a centre drill out. Another counterbore and a single flute boring tool.

[John ReeseParticipant @johnreese12848]

In machine shop practice the amount clearance provided depends on the precision of the hole pattern. In the "old" days for fine work clearance was 1/64" oversize. For ordinary work – 1/32" oversize. For structural work – 1/16" oversize. "Old" is before geometric tolerancing.

[JasonBModerator @jasonb]

And a homemade one for our Aussie members

No fancy grinding machines need to be built, teeth were filed and seem to do the job.

Ian, a quick look and the counterbores with interchangable ends go down to 3/16" head and 3/32" pilot, did not look long enough for metric. The reason for suggesting them to teh OP wa sthat he could make his own pilot to suit his prefered clearance hole which would be easier than grinding an all in one counterbore. They are known as "Aircaft counterbores" and that is also what the OP hinted at when he said he had heard that closer fitting ones are used in aeronautics.

[Ian PParticipant @ianp]

Posted by JasonB on 30/01/2017 20:45:59:

And you could reduce that 0.75mm clearance by 0.4mm if the screw were up tight against one side of the 5.8mm pilot hole so you could be looking at 0.25mm which is not a lot.

J

I think I understand what you mean, but if the shank is uptight against one side of the bolt hole the head cannot then get any closer to the counterbore wall so it does not need extra clearance (other than to allow for head eccentricity).

My interest in the counterbore diameter subject is in regards to small fixings where it seems one would have to make or buy a custom cutter.

Out of interest, I just examined an A2 Stainless M4 caphead. There IS a radius where the shank meets the head but that radius starts at a diameter smaller the M4 thread OD so would not encroach on the bolting face, even if it was an M4 bolt in an 4.00mm hole.

I don't have a problem regarding oversize counterbore pilots or cutters (which some posters seem to infer), I was purely interested 'why' they were so large.

Ian P

[Roderick JenkinsParticipant @roderickjenkins93242]

Posted by Ian Phillips on 30/01/2017 21:48:57:
I would be quite happy to buy a counterbore from ARC but their M4 one is the same size as the one I already have so (JS)I'm not sure what you mean by choice?

**LINK**

Rod

[Ian PParticipant @ianp]

Posted by Roderick Jenkins on 31/01/2017 14:25:24:

Posted by Ian Phillips on 30/01/2017 21:48:57:
I would be quite happy to buy a counterbore from ARC but their M4 one is the same size as the one I already have so (JS)I'm not sure what you mean by choice?

**LINK**

Rod

The M4 counterbore OD in that set is even larger (at 8.5mm) than the other solid counterbore that ARC sell. There is no way I would put an M4 caphead in a 8.5mm counterbore, apart from the crude appearance the counterbore would break out of the edge of the part (say a flange) where the fixings are close to the edge.

The choice is not to purchase but to carry on using the ones I have made, my query was 'why' they were so large.

Ian P

[loco manParticipant @locoman67579]

From a 'Holo-Krome Ltd Metric Series Socket Screw Selector' , the following sizes are listed —

M1,4 – clearance hole 1,6 dia, counterbore for head 2,8 dia.

M1,6 – clearance hole 1,8 dia, counterbore for head 3,3 dia

M2 – clearance hole 2,4 dia, counterbore for head 4,3 dia

M2,5 – clearance hole 2,9 dia, counterbore for head 5 dia.

M3 – clearance hole 3,4 dia, counterbore for head 6 dia

M4 – clearance hole 4,5 dia, counterbore for head 8 dia

M5 – clearance hole 5,5 dia, counterbore for head 10 dia

M6 – clearance hole 6,6 dia, counterbore for head 11 dia

M8 – clearance hole 9 dia, counterbore for head 15 dia

As these sizes are the manufacturer's recommendations, can it be assumed that they allow for any under-head fillet radii? In general, both in model engineering and 'full-size – on steelworks and mining machinery' I have found that metric fasteners both socket head and hexagon to be over-sized (across-flat in particular) and clumsy compared to their near-equivalent imperial sizes. Please, why??

Ian

[Peter SpinkParticipant @peterspink21088]

Posted by John Stevenson on 30/01/2017 22:52:21:

Tool for trepanning the broken bit of a centre drill out.

Excellent, how many times could I have done with one!

[Ian PParticipant @ianp]

Posted by Peter Spink on 02/02/2017 22:32:26:

Posted by John Stevenson on 30/01/2017 22:52:21:

Tool for trepanning the broken bit of a centre drill out.

Excellent, how many times could I have done with one!

Here's an alternative way of dealing with a broken off centre drill.

I actually took the bit I broke, ground the notch with a Dremel and was able to recover the part I was making without wasting more than 10 minutes.

Ian P

[Ian PParticipant @ianp]

Posted by loco man on 02/02/2017 22:14:45:

From a 'Holo-Krome Ltd Metric Series Socket Screw Selector' , the following sizes are listed —

M2,5 – clearance hole 2,9 dia, counterbore for head 5 dia.

M3 – clearance hole 3,4 dia, counterbore for head 6 dia

M4 – clearance hole 4,5 dia, counterbore for head 8 dia

In general, both in model engineering and 'full-size – on steelworks and mining machinery' I have found that metric fasteners both socket head and hexagon to be over-sized (across-flat in particular) and clumsy compared to their near-equivalent imperial sizes. Please, why??

Ian

Something seems to happen (in the progression of OD sizes) between M3 and M4, unless there is an error in the table. 0.60 clearance on M3 and 1.12mm on M4 (by my quick measurements of two hand screws).

Maybe the suggested diameters use a range of 'preferred' sizes or maybe Holo Chrome just looked in a counterbore makers catalogue and used those.

Ian P

[JasonBModerator @jasonb]

Looks like they just settled on nearest whole mm once the size gets to M4 and above, same a hex sizes they are not a direct proportion to the shank

[Mike PooleParticipant @mikepoole82104]

Posted by loco man on 02/02/2017 22:14:45:

I have found that metric fasteners both socket head and hexagon to be over-sized (across-flat in particular) and clumsy compared to their near-equivalent imperial sizes. Please, why??

Ian

The original Whitworth spec had huge heads but they finally made them all one size smaller, metric have a few standards and ISO does seem a bit clumsy wit 13 on an 8mm and 17 on 10mm but the Japanese have 12 on an 8mm and 14 on 10mm which to me are much neater especially on a motorbike. You would think that the head size would have had calculations done to arrive at the optimum size in which case why do the standards differ? Unfortunately just because it is metric does not mean it is standard, European car makers often use a smaller head like VW who use 16 and 18mm. If you are making a metric fastener use whatever size you fancy it probably meets someone's standard.

Mike

[Thomas StauboParticipant @thomasstaubo12021]

I just checked my M5 counterbore. It has a 4.2mm pilot, and a 10mm cutter diameter. Go figure!

It's stamped M5 10 4.2 on the skank, so it's not altered just made for a special purpose.

My first thought is that it's for counterboring a threaded hole, but why would you do that!?

[Neil WyattModerator @neilwyatt]

Posted by Thomas Staubo on 03/02/2017 11:28:01:

I just checked my M5 counterbore. It has a 4.2mm pilot, and a 10mm cutter diameter. Go figure!

It's stamped M5 10 4.2 on the skank, so it's not altered just made for a special purpose.

My first thought is that it's for counterboring a threaded hole, but why would you do that!?

For making the counterbore before you tap M5 fine.

[Ian PParticipant @ianp]

Well as its the tapping drill size for M5 I suppose its possible to counterbore whilst say a cover whilst it is in place, then do the threading and cover clearance hole later, but the 10mm diameter still does not make sense.for a caphead.

Maybe these counterbores are not intended for capheads? I have just measured an M5 Pan head fixing and its 9.65mm OD so would be a nice fit in the 10mm hole.

Ian P

[Brad AmosParticipant @bradamos40390]

Thanks, Locoman,

Your message made me wonder if I could get a neater result by substituting imperial counterbores for the metric ones. Here are my calculations, giving first the metric screw size and maximum recommended cap diameter for a socket cap screw, then the nearest imperial screw size, imperial counterbore designation and diameter of counterbore in millimetres, and finally the clearing drill recommended for normal and close tolerances with the imperial counterbore tool.

M3 5.5mm cap Imp screw #4 Cbore diam. 5.56mm (7/32) clearance 3.2mm (close), 3.45mm (standard)

M4 7.0mm cap Imp screw #6 Cbore diam. 7.14mm (9/32) clearance 3.9-4.3 mm

M5 8.5 mm cap Imp screw #10 Cbore diam. 9.52mm (3/8) clearance 5.2-5.6 mm

M6 10 mm cap Imp screw 1/4" Cbore diam. 11mm clearance 6.8-7.2mm

I got my figures for maximum metric screw head sizes from roymech.co.uk, my US hole and counterbore sizes from rorty.net/holes-and-counterbore-sizes and I converted the US drill sizes using CSGnetwork.com drill size conversions. Before I spend money on any imperial counterbores I would be most grateful to hear from anyone who has tried these substitutions with metric cap head screws. I am very reluctant to let imperial units into my workshop, but the calculations suggest that these counterbores might be worth trying, using the normal clearance drills recommended for the metric screws in my Zeus tables.

[JasonBModerator @jasonb]

M3 into 5.5 will go though I counterbored these with a reground drill with almost a flat end and no pilot, clearance hole is 3.0

[Gordon WParticipant @gordonw]

Of course you may need a counterbore for a hex head and a socket, not just for hex drive keys.

[JasonBModerator @jasonb]

Posted by Gordon W on 03/02/2017 16:09:34:

Of course you may need a counterbore for a hex head and a socket, not just for hex drive keys.

I don't follow

[HOWARDTParticipant @howardt]

He means a hex head as in bolt. Yes there are standards for those, and clearances for spanners.

[MikelkieParticipant @mikelkie]

And why is the shank of live centres so short? one always has to insert something into the sleeve for the drift

to get to it

[Author]

Posts